Switching control circuit

ABSTRACT

A switching control circuit includes an N-channel MOSFET having an input electrode applied with an input voltage and an output electrode connected to one end of an inductor and one end of a rectifying element. The other end of the inductor is connected to a first capacitor. A bootstrap circuit is configured to generate a bootstrap voltage on a second capacitor having one end connected to the output electrode of the N-channel MOSFET. The bootstrap voltage is required when the N-channel MOSFET is turned on. A driving circuit is configured to be applied with a driving voltage corresponding to the bootstrap voltage and turn on/off the N-channel MOSFET to generate an output voltage of a target level on the first capacitor. A clamping circuit is configured to clamp the driving voltage to be at a predetermined level or lower.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Japanese PatentApplication No. 2008-322740, filed Dec. 18, 2008, of which full contentsare incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a switching control circuit.

2. Description of the Related Art

Various types of electronic equipment use a DC-DC converter forgenerating an output voltage of a target level from an input voltage.FIG. 2 is a diagram illustrating a general configuration of a step-downDC-DC converter. A DC-DC converter 100 includes an N-channel MOSFET 110,Schottky barrier diodes 111, 112, an inductor 113, capacitors 114, 115,resistors 116, 117, a control circuit 118, a level shift circuit 119,and an inverter 120.

An input voltage V_(IN) is applied to a drain of the N-channel MOSFET110 through a terminal IN, and when the N-channel MOSFET 110 is turnedon, an input voltage V_(IN) is applied to the inductor 113, thecapacitor 114 is charged, and an output voltage V_(OUT) is raised.Thereafter, when the N-channel MOSFET 110 is turned off, by virtue ofenergy accumulated in the inductor 113, an electric current is passedthrough a loop including the Schottky barrier diode 111, the inductor113, and the capacitor 114, the capacitor 114 is discharged, and theoutput voltage V_(OUT) is lowered. In the DC-DC converter 100, theoutput voltage V_(OUT) is controlled so as to reach the target level byturning on/off the N-channel MOSFET 110 by the control circuit 118 sothat a feedback voltage V_(FB) obtained by dividing the output voltageV_(OUT) by the resistors 116, 117 reaches a predetermined level.

Moreover, the DC-DC converter 100 uses the N-channel MOSFET 110, inwhich ON resistance and loss are smaller than those in a case of aP-channel MOSFET, as a transistor for applying the input voltage V_(IN)to the inductor 113. In a case where the N-channel MOSFET 110 is used assuch, when the N-channel MOSFET 110 is turned on, a voltage of thesource of the N-channel MOSFET 110 gets close to the input voltageV_(IN). Thus, in order to keep the N-channel MOSFET 110 on, a voltagehigher than the input voltage V_(IN) by a threshold voltage V_(TH) ofthe N-channel MOSFET 110 is required to be applied to the gate of theN-channel MOSFET 110. Moreover, in order to keep a state in which the ONresistance of the N-channel MOSFET 110 is sufficiently small, a voltagehigher than the input voltage V_(IN) by the order of 5V, for example, isrequired to be applied to the gate of the N-channel MOSFET 110.

Then, a method of using a bootstrap voltage in order to turn on theN-channel MOSFET 110 is generally employed (See Japanese PatentLaid-Open Publication No. 2008-141832, for example). In the DC-DCconverter 100, a voltage V_(REG) applied to a terminal REG is applied tothe capacitor 115 through the Schottky barrier diode 112 and a terminalBC, so that a bootstrap voltage V_(BT) is generated. Here, assuming thatthe voltage V_(REG) is 5V and each forward voltage of the Schottkybarrier diodes 111, 112 is 0.3V, such a state is considered that theN-channel MOSFET 110 is off and an electric current is passed throughthe loop including the Schottky barrier diode 111, the inductor 113, andthe capacitor 114. In this case, a voltage V_(SW) of a terminal SW is−0.3V, a voltage V_(BC) of a terminal BC is 4.7V, and the bootstrapvoltage V_(BT) across the capacitor 115 results in 5V. Therefore, if theN-channel MOSFET 110 is turned on and the voltage V_(SW) becomes equalto V_(IN), V_(BC)=V_(IN)+V_(ET). Then, the level shift circuit 119performs level shift of a control signal outputted from the controlcircuit 118 based on the voltage V_(BC) and the inverter 120 uses thebootstrap voltage V_(BT) as a driving voltage, and thus, the N-channelMOSFET 100 can be kept on.

In the DC-DC converter 100 as above, the terminal IN and the terminal BCmight be short-circuited due to adhesion of dust or the like. Here,assume such a case that the terminal IN and the terminal BC areshort-circuited when the bootstrap voltage V_(BT) is 5V. When theN-channel MOSFET 110 is turned on, an electric current is passed fromthe terminal IN to the terminal SW through the N-channel MOSFET 110. Forexample, assuming that the input voltage V_(IN) is 15V, the currentflowing through the N-channel MOSFET 110 is 1 A, and the ON resistanceof the N-channel MOSFET 110 is 0.2Ω, the voltage V_(SW) of the terminalSW is 14.8V. At this time, since the terminal IN and the terminal BC areshort-circuited, a current path is formed from the capacitor 115 to theN-channel MOSFET 110, and thus, the capacitor 115 is discharged.

Then, when the capacitor 115 is discharged, the bootstrap voltage V_(BT)is lowered, and thus, the N-channel MOSFET 110 is turned off. Even ifthe N-channel MOSFET 110 is turned off, the inductor 113 tries tocontinue passing the current, and the voltage V_(SW) of the terminal SWbecomes −0.3V. On the other hand, since the terminal IN and the terminalBC are short-circuited, the voltage V_(BC) of the terminal BC is 15V.Therefore, a voltage between the terminal BC and the terminal SW is15.3V. Thus, the voltage of 15.3V is also applied to the inverter 120,and assuming that the withstand voltage of the inverter 120 is 7V, forexample, such a state occurs that the withstand voltage of the inverter120 is exceeded. Moreover, if the voltage applied to the gate of theN-channel MOSFET 110 from the inverter 120 reaches the order of 15V, thewithstand gate-source voltage of the N-channel MOSFET 110 might beexceeded.

SUMMARY OF THE INVENTION

A switching control circuit according to an aspect of the presentinvention, comprises: an N-channel MOSFET having an input electrodeapplied with an input voltage and an output electrode connected to oneend of an inductor and one end of a rectifying element, the other end ofthe inductor connected to a first capacitor; a bootstrap circuitconfigured to generate a bootstrap voltage on a second capacitor havingone end connected to the output electrode of the N-channel MOSFET, thebootstrap voltage required when the N-channel MOSFET is turned on; adriving circuit configured to be applied with a driving voltagecorresponding to the bootstrap voltage and turn on/off the N-channelMOSFET to generate an output voltage of a target level on the firstcapacitor; and a clamping circuit configured to clamp the drivingvoltage to be at a predetermined level or lower.

Other features of the present invention will become apparent fromdescriptions of this specification and of the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For more thorough understanding of the present invention and advantagesthereof, the following description should be read in conjunction withthe accompanying drawings, in which:

FIG. 1 is a diagram illustrating a configuration example of a step-downDC-DC converter including a switching control circuit according to anembodiment of the present invention; and

FIG. 2 is a diagram illustrating a general configuration of a step-downDC-DC converter.

DETAILED DESCRIPTION OF THE INVENTION

At least the following details will become apparent from descriptions ofthis specification and of the accompanying drawings.

FIG. 1 is a diagram illustrating a configuration example of a step-downDC-DC converter including a switching control circuit according to anembodiment of the present invention. A DC-DC converter 10 includes aswitching control circuit 20, a Schottky barrier diode 22, an inductor24, capacitors 26, 28, and resistors 30, 32. The switching controlcircuit 20 includes an N-channel MOSFET 40, a Schottky barrier diode 42,a control circuit 44, a level shift circuit 46, an inverter 48, aresistor 50, and a Zener diode 52.

The switching control circuit 20 is an integrated circuit provided witha terminal REG, a terminal IN, a terminal BC, a terminal SW, and aterminal FB. In the switching control circuit 20, on/off of theN-channel MOSFET 40 is controlled so that a voltage V_(FB) applied tothe terminal FB becomes a predetermined level, and thus, an outputvoltage V_(OUT) at a target level is generated.

In the N-channel MOSFET 40, an input voltage V_(IN) is applied to adrain (input electrode) through the terminal IN, a source (outputelectrode) is connected to the terminal SW, and an output signal of theinverter 48 is inputted to a gate. Therefore, if a potential differencebetween a voltage level of the output signal of the inverter 48 and avoltage V_(SW) of the terminal SW becomes greater than a thresholdvoltage V_(TH) of the N-channel MOSFET 40, the N-channel MOSFET 40 isturned on, and an electric current flows from the terminal IN to theterminal SW through the N-channel MOSFET 40.

The Schottky barrier diode (rectifying element) 22 has an anode groundedand a cathode connected to the terminal SW. The inductor 24 has one endconnected to the terminal SW and the other end connected to one end ofthe capacitor (first capacitor) 26. The other end of the capacitor 26 isgrounded, and a voltage charged in the capacitor 26 is the outputvoltage V_(OUT). Therefore, if the N-channel MOSFET 40 is turned on, theinput voltage V_(IN) is applied to one end of the inductor 24 throughthe terminal SW, the capacitor 26 is charged, and the output voltageV_(OUT) is raised. After that, if the N-channel MOSFET 40 is turned off,by virtue of energy accumulated in the inductor 24, an electric currentis passed through a loop including the Schottky barrier diode 22, theinductor 24, and the capacitor 26, the capacitor 26 is discharged, andthe output voltage V_(OUT) is lowered.

The capacitor (second capacitor) 28 has one end connected to theterminal SW and the other end connected to the terminal BC. Then, thecapacitor 28 is charged with a voltage V_(REG) applied through theterminal BC, to generate a bootstrap voltage V_(BT). This bootstrapvoltage V_(BT) is a voltage used for turning on the N-channel MOSFET 40.For example, it is assumed that the voltage V_(SW) of the terminal SW is0V in an initial state. In this case, if a voltage higher than thethreshold voltage V_(TH) (2V, for example) is applied to the gate of theN-channel MOSFET 40, the N-channel MOSFET 40 is turned on. However, ifthe N-channel MOSFET 40 is turned on, the voltage V_(SW) of the terminalSW gets close to the input voltage V_(IN) (15V, for example), and thus,in order to keep the N-channel MOSFET 40 on, a voltage higher than theinput voltage V_(IN) needs to be applied to the gate of the N-channelMOSFET 40. Therefore, by generating the bootstrap voltage V_(BT) of theorder of 5V, for example, using the voltage V_(REG), the N-channelMOSFET 40 can be turned on even after the voltage V_(SW) of the terminalSW gets close to the input voltage V_(IN).

The resistor 30 has one end applied with the output voltage V_(OUT) andthe other end connected to one end of the resistor 32. The other end ofthe resistor 32 is grounded, and a voltage at connection point of theresistors 30, 32 is a feedback voltage V_(FB) obtained by dividing theoutput voltage V_(OUT) in resistance ratio between the resistors 30 and32.

The Schottky barrier diode 42 (bootstrap circuit) has an anode appliedwith the voltage V_(REG) through the terminal REG and a cathodeconnected to the terminal BC through the resistor 50. Here, assumingthat the voltage V_(REG) is 5V and each forward voltage of the Schottkybarrier diodes 22, 42 is 0.3V, the bootstrap voltage V_(BT), with whichthe capacitor 28 is charged, is considered. If the N-channel MOSFET 40is turned off and the electric current is passed through the loopincluding the Schottky barrier diode 22, the inductor 24, and thecapacitor 26, the voltage V_(SW) of the terminal SW is −0.3V. If voltagedrop at the resistor 50 is ignored, the voltage V_(BC) of the terminalBC is 4.7V, which is lower than the voltage V_(REG) only by 0.3V, whichis the forward voltage of the Schottky barrier diode 42. Therefore, thebootstrap voltage V_(BT) is 4.7−(−0.3)=5V. Thereafter, the N-channelMOSFET 40 is turned on and assuming that the voltage drop of theN-channel MOSFET 40 due to ON resistance is 0.2V, the voltage V_(SW) ofthe terminal SW is 14.8V. As a result, the voltage V_(BC) of theterminal BC results in 19.8V, which is obtained by adding the bootstrapvoltage V_(BT)=5V to 14.8V. Even if the voltage V_(BC) of the terminalBC becomes higher than the voltage V_(REG), the current does not flowback toward the terminal REG from the terminal BC since the Schottkybarrier diode 42 is provided.

The control circuit 44 outputs a control signal to turn on/off theN-channel MOSFET 40 so that the feedback voltage V_(FB) to be applied tothe terminal FB becomes a reference level corresponding to a targetlevel of the output voltage V_(OUT). For example, if the feedbackvoltage V_(FB) is lower than the reference level, the control circuit 44increases a proportion of the N-channel MOSFET 40 being on, while if thefeedback voltage V_(FB) is higher than the reference level, the controlcircuit 44 decreases the proportion of the N-channel MOSFET 40 being on.

The level shift circuit 46 is applied with the voltage V_(BC) of theterminal BC as a voltage on a power source side through the resistor 50and a ground voltage as a voltage on a ground side, and converts acontrol signal based on the voltage V_(REG) outputted from the controlcircuit 44 into a control signal based on the voltage V_(BC).

The inverter 48 (driving circuit) is applied with the voltage V_(BC) ofthe terminal BC as a voltage on the power source side through theresistor 50 and the voltage V_(SW) of the terminal SW as a voltage onthe ground side. That is, the inverter 48 is applied with a drivingvoltage corresponding to the bootstrap voltage V_(BT). On the basis ofthe control signal outputted from the level shift circuit 46, theinverter 48 controls on/off of the N-channel MOSFET 40 by changing avoltage level of the signal inputted to the gate of the N-channel MOSFET40. Specifically, if the signal outputted from the level shift circuit46 is at L level, the inverter 48 outputs an H-level signalcorresponding to the voltage V_(BC) to the gate of the N-channel MOSFET40 so that the N-channel MOSFET 40 is turned on. On the other hand, ifthe signal outputted from the level shift circuit 46 is at H level, theinverter 48 outputs an L-level signal corresponding to the voltageV_(SW) to the gate of the N-channel MOSFET 40 so that the N-channelMOSFET 40 is turned off.

The resistor 50 (current limiting circuit) has one end connected to theterminal BC and the other end connected to a cathode of the Zener diode52. The Zener diode 52 (clamping circuit) has an anode connected to theterminal SW. The Zener diode 52 clamps the driving voltage of theinverter 48 to be at a predetermined level of the order of 6V or lower,for example, even if the terminal IN and the terminal BC areshort-circuited and the bootstrap voltage V_(BT) becomes extremely high,for example. The resistor 50 is used to limit the current flowingthrough the Zener diode 52 so that the clamp voltage by the Zener diode52 is held at a predetermined level or lower, on the basis ofcurrent-voltage characteristics of the Zener diode 52. Depending on thecurrent-voltage characteristics of the Zener diode 52, a configurationmay be made without the resistor 50.

Subsequently, an operation of the DC-DC converter 10 will be described.Description will be made assuming that the input voltage V_(IN) is 15V,the target level of the output voltage V_(OUT) is 5V, the voltageV_(REG) is 5V, the forward voltage of the Schottky barrier diode 22 is0.3V, the threshold voltage V_(TH) of the N-channel MOSFET 40 is 2V, anda breakdown voltage of the Zener diode 52 is 6V.

First, assuming that an operation of the DC-DC converter 10 is startedin a state where the output voltage V_(OUT) and the bootstrap voltageV_(BT) is 0V. At this time, since the bootstrap voltage V_(BT) is lowerthan 2V, the N-channel MOSFET 40 cannot be kept on. The capacitor 28 isapplied with the voltage V_(REG) through the terminal REG, the Schottkybarrier diode 42, the resistor 50, and the terminal BC, and thebootstrap voltage V_(BT) is raised.

If the bootstrap voltage V_(BT) exceeds the threshold voltage V_(TH) ofthe N-channel MOSFET 40, which is 2V, the inverter 48 can turn on theN-channel MOSFET 40 in a case where the control signal from the controlcircuit 44 is at L level. Then, the control circuit 44 changes thecontrol signal so that the feedback voltage V_(FB) becomes the referencelevel corresponding to the target level of the output voltage V_(OUT).

It is assumed that the terminal IN and the terminal BC areshort-circuited in a state where capacitor 28 is charged with thevoltage V_(REG) so that the bootstrap voltage V_(BT) reaches 5V and theN-channel MOSFET 40 is turned on/off.

If the N-channel MOSFET 40 is changed from on to off, the inductor 24tries to continue flowing the electric current, and thus, the current ispassed through the loop of the Schottky barrier diode 22, the inductor24, and the capacitor 26, and the voltage V_(SW) of the terminal SWreaches −0.3V. Since the terminal IN and the terminal BC areshort-circuited, the voltage V_(BC) of the terminal BC reaches 15V.Therefore, the bootstrap voltage V_(BT) across the capacitor 28 becomes15.3V.

On the other hand, the voltage between the power source side and theground side of the inverter 48, that is, the driving voltage of theinverter 48 is clamped to be held at the order of 6V by the breakdownvoltage of the Zener diode 52. At this time, the resistor 50 functionsso as to suppress the driving voltage of the inverter 48 to the order of6V by limiting the current flowing into the Zener diode 52 from theterminal BC.

Therefore, according to an embodiment of the present invention, aswitching control circuit can be provided having a protective functionin a case where the terminal applied with the input voltage and theterminal applied with the bootstrap voltage are short-circuited. Thatis, in the switching control circuit 20, even if the terminal IN and theterminal BC are short-circuited, by virtue of the protective function bythe Zener diode 52, it is prevented that the driving voltage of theinverter 48 exceeds the withstand voltage of the inverter 48. Moreover,since the voltage of the signal outputted from the inverter 48 to thegate of the N-channel MOSFET 40 is similarly clamped, it is alsoprevented that a gate-source voltage of the N-channel MOSFET 40 exceedsthe withstand voltage.

Furthermore, in the switching control circuit 20, since a current amountof the current flowing through the Zener diode 52 is limited by theresistor 50, it is prevented that the clamp voltage of the Zener diode52 becomes excessive.

The above embodiments of the present invention are simply forfacilitating the understanding of the present invention and are not inany way to be construed as limiting the present invention. The presentinvention may variously be changed or altered without departing from itsspirit and encompass equivalents thereof.

For example, the Schottky barrier diode 22 is used as a rectifyingelement when the N-channel MOSFET 40 is off in an embodiment of thepresent invention, but the rectifying element is not limited to that.For example, an N-channel MOSFET which is turned on/off in acomplementary manner with the N-channel MOSFET 40 may be used as arectifying element.

What is claimed is:
 1. A switching control circuit comprising: anN-channel MOSFET having an input electrode applied with an input voltageand an output electrode connected to one end of an inductor and one endof a rectifying element, the other end of the inductor connected to afirst capacitor; a bootstrap circuit configured to generate a bootstrapvoltage on a second capacitor having one end connected to the outputelectrode of the N-channel MOSFET, the bootstrap voltage required whenthe N-channel MOSFET is turned on; a driving circuit configured to beapplied with a driving voltage corresponding to the bootstrap voltageand turn on/off the N-channel MOSFET to generate an output voltage of atarget level on the first capacitor; and a clamping circuit connected inparallel with the second capacitor and configured to clamp the drivingvoltage to be at a predetermined level or lower.
 2. A switching controlcircuit comprising: an N-channel MOSFET having an input electrodeapplied with an input voltage and an output electrode connected to oneend of an inductor and one end of a rectifying element, the other end ofthe inductor connected to a first capacitor; a bootstrap circuitconfigured to generate a bootstrap voltage on a second capacitor havingone end connected to the output electrode of the N-channel MOSFET, thebootstrap voltage required when the N-channel MOSFET is turned on; adriving circuit configured to be applied with a driving voltagecorresponding to the bootstrap voltage and turn on/off the N-channelMOSFET to generate an output voltage of a target level on the firstcapacitor; a clamping circuit configured to clamp the driving voltage tobe at a predetermined level or lower; and a current limiting circuitconfigured to limit a current amount of a current flowing through theclamping circuit from the other end of the second capacitor.
 3. Theswitching control circuit according to claim 2, wherein the clampingcircuit includes a Zener diode having an anode connected to the outputelectrode of the N-channel MOSFET and a cathode connected to a powersource side of the driving circuit; and wherein the current limitingcircuit includes a resistor with one end connected to the other end ofthe second capacitor and the other end connected to the cathode of theZener diode.
 4. A switching control circuit comprising: an N-channelMOSFET having an input electrode applied with an input voltage and anoutput electrode connected to one end of an inductor and one end of arectifying element, the other end of the inductor connected to a firstcapacitor; a bootstrap circuit configured to generate a bootstrapvoltage on a second capacitor having one end connected to the outputelectrode of the N-channel MOSFET, the bootstrap voltage required whenthe N-channel MOSFET is turned on; a driving circuit configured to beapplied with a driving voltage corresponding to the bootstrap voltageand turn on/off the N-channel MOSFET to generate an output voltage of atarget level on the first capacitor; and a clamping circuit configuredto clamp the driving voltage to be at a predetermined level or lower,wherein the clamping circuit includes a Zener diode having an anodeconnected to the output electrode of the N-channel MOSFET and a cathodeconnected to a power source side of the driving circuit.